One important aspect of preparation is a thorough knowledge of the history of your discipline

Soon after I met Watson, I explained to him my profession:

They lay all the evidence before me, and I am generally able, by the help of my knowledge of the history of crime, to set them straight [STUD].

When he expressed wonder at not being able to solve a crime which I had solved, even though he had heard all that I had heard, I pointed out that he was

Without, however, the knowledge of pre-existing cases which serves me so well [NOBL].

Watson subsequently described a number of cases where I was enabled in solving a case by analogy with previous parallel cases.

It reminds me of the circumstances attendant on the death of Van Jansen, in Utrecht, in the year '34. Do you remember the case, Gregson?
No, sir.
Read it up - you really should. There is nothing new under the sun. It has all been done before [STUD].

Or, to use the language that I now prefer, there are only so many stories.

When consulted by Franqois le Villard, I was able to help him solve a crime by referring him to two parallel cases. I pointed out that he had two of the three qualities necessary for the ideal detective: He has the power of observation and that of deduction. He is only wanting in knowledge and, later in the same case, I got some insight into it by parallel cases in India and in Senegambia [SIGN].

When Colonel Damery asked me about Baron Gruber, I knew of course of the case [ILLU]. On finding that Inspector MacDonald did not know of Jonathan Wild, the master 18th century criminal, I said

Mr. Mac, the most practical thing that you ever did in your life would be to shut yourself up for three months and read twelve hours a day at the annals of crime. Everything comes in circles - even Professor Moriarty. Jonathan Wild was the hidden force of the London criminals, to whom he sold his brains and his organization on a fifteen per cent commission [VALL].

Preparation shades into documentation, since my own previous cases are part of the history of crime:

You will find parallel cases, if you consult my index, in Andover in '77 and there was something of the sort at the Hague last year [IDEN].